Orban OPTIMOD 5750 Operating Manual page 53

3-10
Operation
separation when a single-channel composite waveform is clipped to 3 dB depth. To ensure accurate peak control,
the limiter operates at 512 kHz sample rate.
For those who prefer the sound of conventional composite clipping, we also offer a defeatable composite clipper.
This also provides excellent spectral protection for the pilot tone and subcarriers. The composite clipper drives the
"Half-Cosine Interpolation" composite limiter, which serves as an overshoot compensator for the composite clipper
when it is active. (Overshoot compensation necessary to remove overshoots introduced by the pilot- and SCA-
protection filters following the composite clipper.)
Like conventional composite clipping, the "Half-Cosine Interpolation" composite limiter can still cause aliasing
distortion between the stereo main and subchannels. However, this is the inevitable cost of increasing the power-
handling capability beyond 100% modulation above 5 kHz—the characteristic that makes some people like
composite clipping. This exploits the fact that the fundamental frequency in a square wave has a higher peak level
than the square wave itself. However, any process that makes squared-off waveforms above 5 kHz creates higher
harmonics that end up in the stereo subchannel region (23-53 kHz). The receiver then decodes these harmonics as
if they were L–R information and the decoded harmonics appear at new frequencies not harmonically related to the
original frequency that generated them.
While the processing never clips the pilot tone, the extra spectrum generated by the processing can fall into the 19
kHz region, compromising the ability of receivers to recover the pilot tone cleanly. Therefore, the 5750's composite
processor has a 19 kHz notch filter to protect the pilot tone. This filter does not compromise stereo separation in
any way.
We still prefer to use the 5750's main clipping system to do the vast majority of the work because of its sophisticated
distortion-controlling mechanisms. This means that the 5750 does not rely on composite processing to get loud.
Consequently, broadcasters using its left/right-domain AES3 digital output can enjoy the loudness benefits of the
5750's processing—the 5750 gets competitively loud without composite clipping. However, it is also possible to
reduce the drive level to the 5750's left/right domain overshoot compensators and to increase the composite limiter
drive by a corresponding amount.
This arrangement uses the overall composite limiter (with or without the composite clipper's being active) to provide
overshoot compensation. It has a different sound than using the left/right domain overshoot compensators—the
sound is brighter but has more aliasing distortion (as discussed above). If the composite clipper is active, stereo
separation will decrease.
ITU-R 412 Compliance for Analog FM Broadcasts
ITU-R 412 requires the "average multiplex power" to be limited to a standard value. The 5750 contains a defeatable
feedback multiplex power limiter that constantly monitors the multiplex power according to ITU-R 412 standards.
The power controller automatically reduces the average modulation to ensure compliance. It allows you to set the
"texture" of the processing freely, using any preset.
If a given processing setting would otherwise exceed the multiplex power limit, the power controller automatically
reduces the drive to the peak limiting system. This action retains the compression texture but reduces distortion
while controlling multiplex power.